Separation of particles magnetically is well established in the prior art. Initially, such separation was utilized for separation of ferromagnetic materials, and filters have been developed which have become quite effective for accomplishing this end.
One of the best systems heretofore developed for separation of magnetic particles has utilized high gradient magnetic separation. With high gradient magnetic separation, it is possible to separate magnetic particles of small size and, in addition, to also separate very small particles that are only weakly paramagnetic. In high gradient magnetic separation, a matrix of magnetic material having sharp edges (such as, for example, steel wool) is placed in a steady magnetic field so that positive high gradient magnetic fields are created near the sharp edges of the material making up the matrix. When a carrier (liquid or gas), having particles to be separated included therein, is passed through the filter, particles are attracted to the material of the matrix and held there by the established magnetic forces.
In the collection of paramagnetic or ferromagnetic particles in magnetic separation, a gradient is created which forces the paramagnetic or ferromagnetic particles toward a wire in the direction of an applied field. The resulting field distortion is a positive gradient which exerts a force on a magnetized particle. If the particle magnetization is positive, as for ferromagnetic (or ferrimagnetic) particles, the force is directed toward the wire and collection results if the magnetic force exceeds the competing forces (such as, for example, hydrodynamic drag).
With respect to modifying a liquid for magnetic-gravimetric methods--quite distinct from high gradient magnetic separation--the following is referenced: (1) Y. Zimmels and I. Yaniv, IEEE Trans. Magn., Vol. MAG-12, No. 4, pp. 359-368, July 1976; and (2) S. E. Khalafalla, IEEE Trans. Magn., Vol. MAG-12, No. 5, pp. 455-462, September 1976.
With respect to high gradient magnetic separation, the following is referenced: (1) D. R. Kelland, et al., Supervonducting Machines and Devices, S. Foner and B. B. Schwartz (ed), Chapter 10, Plenum Press, New York, 1974; (2) M. E. Arellano, G. Zambrana and C. Soux, Proc. International Tin Symposium, La Paz, Bolivia, November, 1977; (3) D. R. Kelland, IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, pp. 307-310, September 1973; (4) F. E. Lubrosky and B. J. Drummond, IEEE Trans. Magn., MAG-11, p. 1696, 1976; (5) E. E. Lubrosky and B. J. Drummond, IEEE Trans. Magn. MAG-12, p. 474, 1976; (6) C. Cowen and F. J. Friedlaender, IEEE Trans. Magn. MAG-13, p. 1483, 1977; and (7) IEEE Trans. Magn. Vol. MAG-12, No. 5, 1976.
With respect to prior art patents directed at least generally to separation of diamagnetic or nonmagnetic particles, the teachings of such patents appear to fall within two categories: (1) coating or combining non-magnetic or weakly magnetic particles with magnetic particles using solid magnetic particles to collect the desired particles; and (2) magneto-gravimetric methods. Such patents are, for example, U.S. Pat. Nos. 4,125,460; 4,113,608; 4,089,779; 4,087,004; 4,085,037; 4,062,765; 3,929,627; 3,926,789; 3,923,651 and 933,717.
Separation has, however, not been satisfactorily achieved with respect to diamagnetic, non-magnetic or very weakly paramagnetic particles. This is believed to be due to the very small forces (i.e., the small magnetizations), in most cases, on diamagnetic particles even in high gradient magnetic fields.
Recently, consideration of the physics of field gradients has shown that there are regions of negative gradient as well as regions of positive gradient surrounding a ferromagnetic wire in a transverse field, and this has been reported in "Diamagnetic Capture in Single Wire HGMS", IEEE Trans. Magn. MAG-15, No. 6, November 1979 by F. J. Friedlaender, M. Takayasu and T. Nakano.